ＮｂＴｉ極細多芯超伝導線の臨界電流密度とピンニング特性 Ｉ． 熱処理線

Abstract

Critical current density Jc and its temperature dependence in multifilamentary NbTi wires were measured. Further their pinning behavior was examined. Used samples which have been fabricated by a double stacking method are divided into two groups. One group was heat-treated in the fabrication process in order to produce the precipitates of α-Ti and the filament diameter df of NbTi wires ranges from 0.39 to 1.53μm. For the other group, such a heat-treatment was not done and the df of some samples was reduced to the level of below 0.1μm. In this paper, the experimental results on the former group, namely heat-treated (HT) samples with Cu-CuNi mixed matrix or CuNi matrix, are given. The Jc of samples is affected by df. In case of 4.2K, these samples show a slight difference in Jc at high fields. As the temperature rises, the effect of df on Jc becomes remarkable and the Jc-B curves are very different among samples. Normally, samples with small df show low Jc values and reveal a rapid decrease in Jc with increasing magnetic field. An anomalous change in Jc at low fields of below 0.1T was seen at temperatures close to Tc for samples with sub-micron sized filaments. The pinning force Fp vs. reduced field b(=B/Bc2) curves at 4.2K for different samples coincide with each other except for the sample with df=0.39μm. The reduced pinning curves (Fp/Fp, max vs. b) obtained at various temperatures have similar shapes for each sample. The bm where the maximum of Fp takes place is around 0.35 except for the sample with df=0.39μm. The pinning properties of these samples are discussed in terms of their upper critical field Bc2 and transition temperature Tc.